Multiple beam radio frequency apparatus having cooperating resonators and mode suppression means



April 1965 T. ca. MIHRAN ETAL 3,248,593

MULTIPLE BEAM RADIO FREQUENCY APPARATUS HAVING COOPERATING RESONATORSAND MODE SUPPRESSION MEANS Filed Feb. 16, 1962 2 Sheets-Sheet 1 E PO ER8 HEATER JUPPLY [nverv tor-s,-

LOWE? Theodor-e 6-. Mihr-a r2,

6 Md/co/m 7%. Boyd,

PHASE SH/FT PER SECTION A n Aplll 1966 T. G. MIHRAN ETAL 3,248,593

MULTIPLE BEAM RADIO FREQUENCY APPARATUS HAVING COOPERATING RESONATORSAND MODE SUPPRESSION MEANS Filed Feb. 16, 1962 2 Sheets-Sheet 2 MAX [r7ventor's: Theodore G: M/hr-dn,

Md/cp/m Bg d,

United States Patent MULTIPLE BEAM RADIO FREQUENCY APPARA- Thisinvention relates to multiple-beam radio frequency (r.f.) apparatusadapted for generating and handling relatively 'high electromagneticwave power at relatively high frequencies and more particularly to thesuppression of undesired modes of operation in such apparatus.

In copending US. application S.N. 173,724 of M. R. Boyd et al. filedconcurrently herewith and assigned to the same assignee as the presentinvention, there is disclosed and claimed multiple-beam radio frequencyapparatus which is adapted for generating and handling substantiallyhigh electromagnetic wave power at microwave frequencies and in a mannereffective for attaining maximum-etficiency energy exchange between thebeams and electromagnetic waves in cooperating resonators and forminimizing mode interference problems of the type theretoforeencountered in multiple-beam devices. lieved that by following theteachings of Boyd et al. power output increases of one order ofmagnitude can be obtained over prior microwave power generating devices.However, it is also believed that an attempt to increase the output totwo or more orders of magnitude, which would involve the use of ahundred electron beams or more, would lead to interference betweenadjacent modes.

The present invention contemplates the provision of multiple beam radiofrequency apparatus constructed according to the mentioned Boyd et a1.invention and further constructed to include new and improved means forsuppressing undesired modes and thus is adapted for making realizablepower increases of more than one magniude.

Accordingly, it is an object of this invention to provide new andimproved multiple-beam radio frequency apparatus adapted for increasedmicrowave energy generating and handling capacities.

Another object of this invention is to provide multiplebeam radiofrequency apparatus including new and improved means for suppressingoperation thereof in undesired modes.

Another object of this invention is to provide multiplebeam radiofrequency apparatus adapted for maximumefliciency energy exchangebetween the beams and electromagnetic waves in cooperating resonatorsthereof and means whereby undesirable modes are attenuated withoutadversely affecting such maximum efliciency energy exchange.

Another object of this invention is to provide multiplebeam radiofrequency apparatus adapted for attenuating undesired modes withoutadversely affecting the desired mode of operation or the amount offrequency separation between that mode and adjacent undesired modes.

Other objects and advantages of this invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize this invention will be pointed out with particularity inthe claims annexed to and forming part of this specification.

In carrying out the objects of this invention, and according to oneembodiment thereof, there is provided mul tiple-beam radio frequencyapparatus comprising input, output and preferably at least oneintermediate, longitudinally-resonant waveguides supported in spacedparallel relation. Extending perpendicular to and in cooperativeassociation with the waveguides are a plurality It is be-' of parallelklystron-like beam devices. Each such device includes a plurality ofaxially-spaced drift tubes defining input, output, and one or moreintermediate, interaction gaps each located in a respective one of thementioned waveguides, an electron gun for projecting a beam of electronsthrough the drift tubes past the interaction gap and a collector forcollecting electrons emerging from the last drift tube. In eachwaveguide the interaction gaps, defined by the opposed ends of adjacentdrift tubes, comprise equally-spaced active capacitive elements, andinterposed midway between each pair of active elements therein is apassive, or dummy, capacitive element having a capacitance valuesubstantially equal to that of an active element. Further, the periodicelectrical spacing between adjacent capacitive elements and between theoutermost capacitive element and adjacent end walls in each waveguide ismade equal to one quarter of the loaded guide wavelength at apredetermined operating frequency. Suitable means is provided forexciting the input waveguide to establish therein a standingelectromagnetic wave of the aforementioned frequency which results inthe occurrence therein of an electric field maximum at each activecapacitive element in the input waveguide and a voltage node at eachpassive capacitive element. Thusly, and in accordance with the inventionof the above-mentioned Boyd et a1. application, the apparatus is adaptedfor cooperative maximum-efficiency energy exchange between the wave inthe input section and all of the beams passing therethrough foreffecting velocity modulation of the electrons in the beams whichresults in the electrons becoming density modulated in the subsequentfield-free drift regions. The density-modulated electrons coopera tivelyexcite similar standing waves in the intermediate waveguides whichresults in further density modulation of electrons in subsequent driftregions and, finally, the density-modulated electrons cooperatively, andwith maximum-efiiciency energy exchange, induce a correspondingamplified electromagnetic wave in the output waveguide wherein theelectric field maxima occur at the active gaps and the voltage nodes atthe passive elements. The electromagnetic wave energy is extracted fromthe output waveguide by any suitable means. When the device is operatedso that the mentioned field maxima and minirna occur, respectively, atthe active and passive gaps the device is adapted for 1r/ 2 mode whichis desirable in that in such operation the mentioned maximum-efficiencyenergy exchange and maximum mode separation are attained. In order toattenuate any undesired modes without adversely affecting the advantagesobtainable in 1r/2 mode there is provided, according to the presentinvention, resistive elements at the same locations as the passivecapacitive elements. Thus, at such location there is provided impedancemeans, each constituting a combination of a capacitive reactance and aresistance; and when the apparatus operates in the desired 1r/2 mode, avoltage node occurs at the passive impedance means and the resistanceelement will have no effect on the standing waves and resonators.However, if the apparatus should tend to operate in an adjacentundesired mode, or one other than 1r/2, the undesired mode will haveelectric field components occurring at each of the passive impedancemeans and the resistive elements thereof will attenuate the electricfield components, whereby the undesired mode will be effectivelyattenuated without adversely affecting the desired rr/ 2 mode ofoperation. The passive impedance means, whereby combined resistance andcapacitance components are provided, can assume various alternativeforms according to the present invention. For a better understanding ofthe invention reference may be had to the accompanying drawing in which:

FIGURE 1 is a sectional view of a multiple-beam electric dischargedevice constructed according to one embodiment of the invention;

FIGURE 2 is a sectional view taken along the lines 22 in FIGURE 1 andlooking in the direction of the arrows;

FIGURE 3 is a sectional view taken along the lines 3--3 in FIGURE 1 andlooking in the direction of the arrows;

FIGURE 4 is an W-B diagram illustrating the maximum mode separationattainable in 1r/ 2 operation;

FIGURE 5 is a schematic illustration of the electric field distributionin the apparatus of FIGURE 1 in 1r/ 2 operation;

FIGURE 6 is a schematic illustration adapted to show the attenuatingeffects of the periodic passive impedance means in the present structurewhen the apparatus tends to operate in other than 7r/ 2 mode;

FIGURE 7 is a detailed illustration of a specific form of combinedresistance and capacitive reactance impedance means employable accordingto the present invention; and

FIGURE 8 is a modified form of combined resistance and capacitivereactance impedance means also employable in the present invention.

Referring now to FIGURE 1, there is shown multiplebeam radio frequencyamplifying apparatus constructed in accordance with the invention. Morespecifically, the arrangement of FIGURE 1 is an electric dischargedevice in which D.C. energy from four electron beams is converted intoelectromagnetic wave energy to afford a multiple-beam device havingsubstantially four times the power generating and handling capacities asa single-beam klystron of comparable beam. dimensions and which is,according to the present invention, adapted for attenuating orsuppressing other than 1r/2 modes. However, from the outset, it is to beunderstood that the modesuppressing means to be disclosed in detailhereinafter can be used with multiple-beam devices having more or lessthan four electron beams.

The device of FIGURE 1 is constructed as a unitary evacuated envelopecomprising four longitudinally resonant waveguides designated 1-4arranged in spaced parallel relation and a plurality of transverselyextending, equally-spaced cooperating klystron-like beam devicesdesignated 58. In this arrangement, and according to the invention ofthe above-referenced Boyd et a1. application, each of the waveguides 1-4is a short-circuited or longitudinally resonant section of aperiodically-loaded waveguide. The waveguides are preferably rectangularin cross-section as shown; however, it is to be understood and it willbe appreciated from the following disclosure, that the invention is notlimited to use of waveguides of this particular cross-sectionalconfiguration and, in fact, in some applications resonant sections ofother forms of transmission lines, such, for example, asline-over-groundplane transmission lines, are employable.

The lowermost waveguide 1 in FIGURE 1 constitutes an input resonator andis adapted to be excited for having a standing electromagnetic waveestablished therein by any suitable radio frequency input couplingmeans, such as an inductive coupling loop 11 seen in FIGURES 2 and 3. Ina manner generally similar to that well known in the klystron art, theinput resonator is effectively employed to velocity modulate theelectrons in the beams of the devices 5-8. The uppermost Waveguide 4 inFIGURE 1 constitutes an output resonator and is adapted for having anamplified electromagnetic wave induced therein through the cooperativeinteraction of all of the beams and the wave established in the outputresonator. Energy is extracted from the output resonator by any suitableradio frequency output means such as an inductive coupling loop 12 (FIG.3). Interposed between the input and output resonators 1 and 4 areintermediate resonators 2 and 3 which are shown as two in number butwhich can be employed in any desired number. The

intermediate resonators serve to increase beam modulation and bunchingefiiciency in generally the same wellknown manner as intermediateresonators found in the klystron art.

The frequency characteristics of each of the resonators is selectivelyvariable to some extent by adjustable short circuits in the form ofsliding conductive end members 15 which, as seen in FIGURE 1, can belocated at both opposed ends of each waveguide and can be provided withflexible hermetic sealing means 16 to maintain a vacuum in thestructure.

In accordance with the above-mentioned invention of Boyd et al., thewaveguides 1-4 are each periodicallyloaded with alternate active andpassive capacitive elements. More specifically, the beam devices 5-8each include a plurality of axially-spaced and aligned tubular sections17 each having at least one end thereof sealed and extending reentrantlyin one of the waveguides. In the waveguides the opposed ends of sections17 define active capacitive gaps 20 and the ones of the sectionsextending between waveguides constitute drift tubes. An emittergenerally designated 18 in each of the gun sections of the devices 5-8projects a beam of electrons through the aligned section 17 and acrossthe several interaction gaps 20, which electrons are finally collectedin a collector 21 sealed to the endmost tubular section 17. Theresonator assemblies are surrounded by solenoid coil 22 to provide acollimating magnetic field for focussing the electron beam. The coil 22is enclosed in a casing 23 formed of a material of low reluctance, such,for example, as soft iron, to provide a uniform axial magnetic fieldwithin the region through which the electron beams pass. The electronguns 18, which can be positioned outside the casing 23, can be suppliedwith operating potentials by any suitable means known to those skilledin the art and generally indicated in FIGURE 1.

The active gaps 20 have uniform capacitance values across each waveguideand, as seen in the drawing, are periodically, or uniformly equallyspaced, along each waveguide. Also in accordance with the mentioned Boydet al. invention there is provided midway between each pair of adjacentactive gaps in each resonator a passive, or dummy, capacitance 24 ashereinafter described having a value substantially the same as thecapacitance value of one of the active gaps 20, and the outermost onesof the gaps 20 are spaced from the effective end walls of the resonatorsby amounts equal to the spacing between adjacent active and passivegaps. Thus, the present device is adapted for affording the operatingadvantages obtainable with the structure disclosed and claimed in theBoyd et al. application when that structure is operated in 1r/2 mode.More specifically, in 1r/2 operation the field maxima and minima occur,respectively, at the active and passive gaps in each waveguide whichresults in maximum-efficiency energy exchange between the electron beamstraversing the active gaps and waves in the waveguides. Also, and asshown in FIGURE 4, in 1r/ 2 operation there is maximum frequencyseparation between the 1r/2 mode and adjacent modes which is desirablein that it minimizes any tendency for the apparatus to operate inadjacent undesired modes. Expressed in another manner, while the 1r/ 2mode of operation provides maximum frequency separation of adjacentmodes, the frequency separation of these adjacent modes in deviceshaving large numbers of beams may be undesirably close. For example, abeam circuit will have 100 possible modes of operation spaced only 1% orless apart in frequency. Inasmuch as the output cavity for suchklystrons may typically have a loaded Q in the hundreds, the half powerpoint of the desired 1r/2 mode may well correspond with the half powerpoint of an adjacent mode and the device may tend to operate in thisadjacent mode. The present structure is adapted for attenuating andsuppressing modes other than 1r/2 mode and thus further minimizing anytendency for the apparatus to operate in the mentioned adjacent modesand thereby enabling use of greater numbers of electron beams. This isaccomplished by providing at the points midway between the active gapspassive impedance means generally designated 24 in FIGURE 1 and adaptedfor providing both a resistive element as Well as the capacitancerequired for the practice of the invention of Boyd et al. Morespecifically, and according to the present invention, each of theimpedance means 24 constitutes a parallel combination of a capacitivereactance, or the passive capacitive gap referenced above, substantiallyequal in capacitance value to the capacitance of the active gaps and aresistance element adapted for high electrical loosiness.

In ordinary operation in the desired 1r/2 mode the mode pattern in eachwaveguide 1-4 is as illustrated in FIGURE 5 and the field maxima andminima occur, respectively, at the active gaps 20 and the impedancemeans 24. includes capacitive reactance substantially equal to that ofthe active gaps 20, the apparatus is adapted for operating so as toattain the advantages of the Boyd et al. invention. However, should theapparatus tend, for example, to operate in the next adjacent mode, suchas the 31/8 mode, in the manner shown in FIGURE 6, the voltage nodeswill not coincide with impedance means 24 with the result that currentswill flow across the resistive elements thereof and thereby introducelosses at these points. As seen in FIGURE 6, the losses introduced bythe resistive elements causes the voltage wave form to be attenuatedsubstantially in the 31r/8 mode. The same would occur at the 51r/8 modeor any mode other than 1r/2.

Thus, in 1r/2 operation of the presently disclosed structure theresistive elements of the impedance means 24 have no effect on theabove-discussed desired operation of the apparatus in view of theappearance of voltage node-s at these points. However, should theapparatus tend to operate in other than 1r/ 2 mode the voltage nodes andresistive elements will not coincide, with the desired result thatelectrical losses will be introduced which will have the desired efiectof attenuating, 'or damping, the other than 1r/ 2 mode. Also, and asindicated above, this desired attenuation is attained withoutsubtracting from the capabilities of the apparatus in realizing theadvantages discussed above and obtainable with the Boyd et al.invention.

In FIGURE 1 the means 24 are generally indicated and, according to thepresent invention, any suitable means effective for providing therequired parallel combination of resistance and capacitive reactance canbe employed as the mentioned passive impedance means. However, in FIGURE7 is illustrated a waveguide res-onator incorporating a particularembodiment of impedance means employ-able in the present invention andeffective for providing the desired combination of electricalcharacteristics. In this embodiment a waveguide, which can be any one ormore of those designated 1-4 in FIGURE 1, is periodically loaded withalternate active capacitive gaps 20 and impedance mean-s generallydesignated 25. The passive impedance means in this form each comprise aconductive member 26 having a relatively high resistance conductivemember 27 interposed between each end thereof and the adjacent walls ofthe waveguide. If desired, the members 26 can comprise copper posts andthe members 27 can comprise ceramic disks impregnated with ahigh-resistance conductive material. The conductive members 26 and thespaces between the ends thereof and the waveguide walls are such as toprovide the desired passive capacitance which is substantially equal tothe capacitance of active gap 20. Additionally, the combined resistanceprovided by the members 26 and 27 in each passive impedance means issuch as to satisfy the requirements for an electrically lossy elementlocated midway between each adjacent pair of active gaps.

Inasmuch as the impedance means 24 each It will be understood from theforegoing that, if desired, the two resistive members 27 can be replacedby a single member disposed between one end of each of the members 26and one wall of the waveguide providing'the resistance characteristicaiiorded thereby is satisfactory for introducing the proper amount oflossiness to modes other than 1r/2. Also, if desired, the disk-likemembers can, if appropriately dimensioned, define the capacity gap andhave a high-resistance conductive member extending therebetween toprovide the required resistance interconnection.

Illustrated in FIGURE 8 is another form of the present invention adaptedfor providing the required combined resistance and capacitivecharacteristics at the locations midway between the active gaps. In thisembodiment the waveguide which also can be any one or more of thosedesignated 1-4 in FIGURE 1, is periodically loaded with alternate activegaps 20 and impedance means generally designated 30. The impedance means30 each comprise a pair of opposed cylindrical conductive members 31which cooperate to provide a passive capacitive gap having substantiallythe same capacitance value as one of the active gaps 20. Extendingcoaxially in the members 31 and electrically interconnecting the opposedthe active gaps and, for example, can comprise the same material ofwhich the disks 27 in FIGURE 7 are formed. In this embodiment each ofthe impedance means 30 can, if desired, comprise only one cylindricalmember 31 provided the latter extends an appropriate distance from theactive waveguide wall and is of appropriate dimensions to aiford therequired capacitance value. Also, it will .be appreciated, that ifdesired, the members 32 can be tubular with one or more of the members31 extending therein.

It is to be understood from the foregoing that the present invention isnot limited to unitary evacuated devices such as that illustrated in thedrawing. The invention is equally applicable to apparatus such as thatdisclosed as the second embodiment of the mentioned Boyd et al.application and wherein the beam devices and wave-' guide resonatorscomprise discrete subassemblies with the beam devices detachably mountedin or coupled to external resonant transmission line or waveguidesections. In such an arrangement the tuning members and the passiveimpedance means would be mounted in the external waveguide ortransmission line sections.

Further, the present invention is not limited to apparatus wherein theactive capacitive gaps comprise the interaction gaps of beam-typedevices. The present concept of periodically loading a resonant sectionof transmission line with alternate active and passive impedance meansand wherein the passive impedance means includes resistive elements forattenuating undesired modes is applicable also to apparatus whereinother types of active gaps are employed, such, forexample, as theapparatus disclosed and claimed in the copending US. application S.N.173,703 of R. A. Dehn, filed concurrently herewith and assigned to thesame assignee as the present invention and wherein space-chargecontrolled devices are employed.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A multiple beam radio frequency apparatus comprising at least onelongitudinally resonant section of transmission line periodically loadedby a longitudinally extending array of alternate active and passiveimpedance means of substantially equal capacitance values at theoperating frequency of said apparatus, said active impedance meansconstituting interaction gaps, said impedance means being electricallyspaced M4 length of wave energy in said waveguide when said waveguide isloaded for effective 1r/2 mode operation, means directing electronsacross said interaction gaps for enabling energy exchange between saidelements and an electromagnetic wave on said line, and said passiveimpedance means each including means providing a capacitive reactancesubstantially equal in value to the capacitance of said interaction gapsand an adjacent high resistivity conductive path across said line saidpassive impedance means acting as mode suppression means in saidapparatus for other than the 11/ 2 mode.

2. A multiple beam radio frequency apparatus comprising at least a pairof spaced longitudinally-resonant waveguides each periodically loaded bya longitudinallyextending array of alternate equally-spaced active andpassive impedance means said means being electrically spaced /4 thelength of wave energy in said waveguide when said waveguides are loadedat a predetermined operating frequency at the 1r/2 mode, the activeimpedance means of each waveguide constituting interaction gaps andbeing aligned with respective gaps of the other waveguide, meansdirecting electrons successively across the respective gaps of firstsaid one and then the other waveguide, and said passive impedance meansin at least one of said waveguides each comprising an adjacent parallelcombination of a capacitive reactance equal in capacitance value to oneof said interaction gaps at the operating frequency of said apparatusand a high-resistivity conductive path between opposed sides of saidwaveguide.

3. Multiple-beam radio frequency apparatus comprising at least a pair ofspaced longitudinally-resonant waveguides each periodically loaded by alongitudinally extending array of alternate equally-spaced active andpas sive impedance means being spaced A, the length of wave energy insaid waveguide when said waveguides are loaded at a predeterminedoperating frequency in the 1r/2 mode, said active impedance means ofeach waveguide constituting interaction gaps of substantially equalcapacitance values and being aligned with respective gaps of the otherwaveguide, said passive impedance means each including a capacitiveelement having a value substantially equal to the capacitance value ofone of said interaction gaps at the operating frequency of saidapparatus, means for establishing in one said waveguides a standingelectromagnetic wave having electric field maxima and minima occurring,respectively, at said active and passive impedance means, means forprojecting a plurality of discrete electron beams across saidinteraction gaps in first said one and then the other said waveguidesand including drift-space-defining means located along said beamsbetween said interaction gaps, whereby an amplified electromagnetic waveis induced in said other waveguide corresponding to said wave in saidone waveguide and having electric field maxirna and miniina occurring,respectively, at said active and passive impedance means, said passiveimpedance means each further including an adjacent relativelyhigh-resistivity conduction path extending across said waveguide,whereby said passive impedance means are effective for attenuating modeshaving voltage minima occurring other than at said passive means, andmeans for extracting radio frequency energy from said other waveguide.

4. In a multiple-beam radio frequency apparatus comprising alongitudinally resonant waveguide, said waveguide being periodicallyloaded by equally-spaced means defining alternate active and passivecapacitive gaps between opposed walls of said Waveguide, said gaps beingelectrically spaced the length of wave energy in said waveguide whensaid Waveguide is loaded at a predetermined 1r/2 mode, said gaps havinga capacitive value essentially equal at the operating frequency of saidapparatus, said active gaps constituting interaction gaps adapted forhaving discrete electron beams projected thereacross inenergy-exchanging relation with an electromagnetic wave in saidwaveguide, the improvement which comprises means in parallel with eachsaid passive gaps providing a high-resistivity conduction path betweensaid opposed walls of said waveguide and effective for attenuating andsuppressing radio frequency energy of all modes except that havingvoltage nodes occurring at said passive gaps.

5. In a multiple-beam radio frequency apparatus comprising alongitudinally resonant waveguide, said waveguide being periodicallyloaded with equally-spaced active and passive impedance means betweenopposed walls of said waveguide, said means being electrically spacedalong said waveguide A1 length of wave energy in said waveguide whensaid waveguide is loaded for operation in the 1r/2 mode, said activemeans constituting intermediate gaps adapted for having discreteelectron beams projected thereacross in energy-exchanging relation withan electromagnetic wave in said waveguide, the improvement whichcomprises said passive means each comprising a conductive memberextending between said opposed walls of said waveguide and separatedtherefrom by at least one intermediate relatively high-resistivityconductive member, whereby a capacitive gap and a highresistivityconduction path are provided, the capacitance of said capacitive gapbeing substantially equal to the capacitance of said active impedancemeans at the operating frequency of said apparatus.

6. In a multiple-beam radio frequency apparatus comrising alongitudinally resonant waveguide, said waveguide being periodicallyloaded by equally-spaced alternate active and passive impedance meansbetween opposed walls of said waveguide, said means being electricallyspaced At length of Wave energy in said waveguide when said waveguide isloaded for operation in the rr/Z mode, said active means constitutinginteraction gaps adapted for having discrete electron beams projectedthereacross in energy-exchanging relation with anrelectromagnetic wavein said waveguide, the improvement which comprises said passive meanseach comprising a conductive member extending between said opposed wallsof said waveguide and having each end thereof separated from saidWaveguide by an intermediate relatively high-resistivity conductivemember, whereby capacitive gaps and a high-resistivity conduction pathare provided, the capacitance of said capacitive gaps beingsubstantially equal to the capacitance of said active impedance means atthe operating frequency of said apparatus.

7. in a multiple-beam radio frequency apparatus comprising alongitudinally resonant waveguide, said Waveguide being periodicallyloaded by equally-spaced alternate active and passive impedance meansbetween opposed walls of said waveguide, said means being electricallyspaced A length of wave energy in said waveguide when said waveguide isloaded for operating at a predetermined 1r/ 2 mode, said active meansconstituting interaction gaps adapted for having discrete electron beamsprojected thereacross in energy-exchanging relation with anelectromagnetic wave-in said waveguide, the improvement which comprisessaid passive means each comprising at least one conductive memberextending from one said opposed walls of said waveguide toward the otherfor defining therewith a capacitive gap having a capacitancesubstantially equal to the capacitance of said active impedance means atthe operating frequency of said apparatus, and a cooperating relativelyhigh-resistivity conductive member electrically interconnecting saidopposed walls.

8. In a multiple-beam radio frequency apparatus comprising alongitudinally resonant waveguide, said waveguide being periodicallyloaded by equally-spaced alternate active and passive impedance meansbetween opposed walls of said waveguide, said means being electricallyspaced At length of wave energy in said waveguide when said wave uide isloaded for operation in the 7r/2 mode, said active means constitutinginteraction gaps adapted for having discrete electron beams projectedthereacross in energy-exchanging relation with an electromagnetic wavein said Waveguide, the improvement which comprises said passive meanseach comprising at least one tubular conductive member extending fromone said opposed walls of said waveguide toward the other for definingtherewith a capacitive gap having a capacitance substantially equal tothe capacitance of said active impedance means at the operatingfrequency of said apparatus, and a coaxial relatively high-resistivityconductive member electrically interconnecting said opposed walls.

9. In a multiple-beam radio frequency apparatus comprising alongitudinally resonant waveguide, said wave guide being periodicallyloaded by equally-spaced alternate active and passive impedance meansbetween opposed walls of said waveguide, said means being electricallyspaced in said waveguide 4 length of wave energy in said waveguide whensaid waveguide is loaded for operation in the 1r/2 mode, said activemeans constituting interaction gaps adapted for having discrete electronbeams projected thereacross in energy-exchanging relation withanelectromagnetic wave in said waveguide, the improvement of saidpassive means each comprising a pair of coaxial conductive membersextending from said opposed walls of said waveguide and defining acapacity gap having a capacitance substantially equal to the capitanceof said active impedance means, at the operating 10 frequency of saidapparatus, and a coaxial relatively highresistivity conductive memberelectrically interconnecting said opposed walls.

References Cited by the Examiner UNITED STATES PATENTS 2,458,556 1/1949Bowen 3155.46 X 2,515,225 7/1950 Holst et al. 33383 X 2,682,641 6/1954Sensiper 33383 X 2,745,910 5/1956 Dehn 33383 X 2,830,224 4/1958 Jenny33383 X 2,875,376 2/1959 Havstad 33383 X 2,899,647 8/1959 Willwacher33383 X 2,901,660 8/1959 Pearce et al 333-83 X 2,910,614 10/1959 Bondley333-83 X 2,920,229 1/1960 Clarke 3155.16 2,934,672 4/1960 Pollack et al333-83 X 2,944,233 7/1960 Fong 33383 X FOREIGN PATENTS 686,830 2/1905Great Britain.

HERMAN KARL SAALBACH, Primary Examiner. ARTHUR GAUSS, Examiner.

c.o. GARDNER, s. CHATMON, ]R.,

Assistant Examiners.

1. A MULTIPLE BEAM RADIO FREQUENCY APPARATUS COMPRISING AT LEAST ONELONGITUDINALLY RESONANT SECTION OF TRANSMISSION LINE PERIODICALLY LOADEDBY A LONGITUDINALLY EXENDING ARRAY OF ALTERNATE ACTIVE AND PASSIVEIMPEDANCE MEANS OF SUBSTANTIALLY EQUAL CAPACITANCE VALUES AT THEOPERATING FREQUENCY OF SAID APPARATUS, SAID ACTIVE IMPEDANCE MEANSCONSTITUTING INTERACTION GAPS, SAID IMPEDANCE MEANS BEING ELECTRICALLYSPACED 1/4 LENGTH OF WAVE ENERGY IN SAID WAVEGUIDE WHEN SAID WAVEVGUIDEIS LOADED FOR EFFECTIVE /2 MODE OPERATION, MEANS DIRECTING ELECTRONSACROSS SAID INTERACTION GAPS FOR ENABLING ENERGY EX-